New Type of Invisibility Cloak?

Yet again we’ve had invisibility cloaks in the news recently and I’m not talking about the recent release of the new Harry Potter movie.

This has been in the news a lot in recent years but there’s a new twist on this idea that’s both interesting and frustrating

This, of course all boils down to a type of construct called meta-materials. These are engineered materials, sometimes called nano-metric which can interact with light in such a way that it guides it along its surface. If done properly, light will exit the material in such a way that it will continue on the same path the light would have taken if the metamaterial were not there. Anyone looking at this material then would not see that the light took a detour around anything. Hence we have an invisibility cloak….of sorts anyway. Up til now only very limited frequencies can be used like microwaves and only little things can be cloaked. Still, the day may come when large objects can be made invisible to visible light waves. I think we’re only scratching the surface of what metamaterials can do.

Case in point…

Martin McCall, an optical physicist at Imperial College London who led the team behind the new proposal said

“I realized that it may be possible to use metamaterials to bend light rays in both space and time, not just in space…This would add a new dimension to the invisibility cloak — literally.”

The new bit then is not to hide an object but to hide events. Kind of like creating a space-time bubble around an event so that for a period of time it cannot be observed. McCall’s example involves robbing a safe:

He says: “You could imagine a burglar using a space–time cloak to create an invisible corridor leading to a safe,”

The thief could then walk to the safe, open it and take all the money, then leave, all with out the chance of being detected because he could not be observed.

So how can this be pulled of??

This might be accomplished using a material or process that can quickly manipulate the index of refraction of the material. This index determines how fast light travels in a medium. Water has a higher Index of refraction than space because light slows down in water. Events can be made unobservable then by manipulating the speed of light; speeding up some light and slowing down other light. It’s this gap in between where events can happen unnoticed.

For example, the reflected light showing an unmolested safe would be slowed down; the final bit of light showing the safe after the robbery would be sped up and stitched together with the initial light that was slowed down. This then edits out the robbery preventing that event from being observed.

Being able to hide events such as this sure could come in handy but what does the theory itself say about how possible this scenario might be? Well let’s see…

1- A space-time void of even just 3 minutes would require a cloak that is bigger than the earth.

Oooookay….

2. The theoretical calculations only work in a vacuum.

???

3. The theory requires the metamaterial to boost the speed of light beyond its maximum speed.

Wow! Really?!?

What the hell….

Why do they get my hopes up if the problems are that significant? Give me a break.

To make it worse there are easy workarounds…..for example….just put a clock near the thing you’re observing. I assume if someone tried to pull a cloaked fast-one you’d see the clock slow down and then speed up unaccountably. As soon as that clock slows down….blam…spray riot foam everywhere and sort it out later.

Also, you need a metamaterial around an area that you want to hide events in. Setting this up would be noticed I would think.

So can this be good for anything? Well of course it can.

It turns out that we may be able to pull off this cloak within fiber optics. In this medium, the speed of light does not need to be exceeded. The trade-off is that the event isn’t strictly 100% unabservable from every angle but it can still have some interesting computer applications. For example, quantum computers. Manipulating quantum states are difficult because they are so fragile. Any interaction with the environment (called dechoherence) and your superpositioned calculations collapse. Anything that can isolate your quantum states from potential observation could be very helpful in a quantum computer.

There’s even some more potential computer applications that also don’t require a vacuum or faster than light light. So I guess that’s good.

I often like to point out news articles that use bad science or bad analogies in their titles.

In this case it was a comparison to Star Trek technology. Here’s one title:

‘Spacetime cloak’ could act as ‘Star Trek transporter’

That’s just silly. Sure you can imagine this tech hiding a person traveling from A to B. He would seem to disappear from one spot and appear a moment later in another spot. Does that mean you can compare it to turning a person into energy, beaming them to another place and reassembling them perfectly? You decide.

3 comments to New Type of Invisibility Cloak?

Definitely interesting and there are obvious problems as you mention. There are a couple of things I find slightly off/interesting that I thought you might appreciate:

First, I don’t get the impression that the calculations only work in a vacuum. In fact, the original paper goes over a second set of calculations where the background medium is non-vacuum.

Second, the theory would only require a metamaterial to boost the speed of light faster than c IF you wanted to create this cloaking effect in a vacuum, i.e. the bank robbery. You could still have the light off the safe passing through several mediums where the light can be slowed down and sped up, all obeying modern physics principles, but it just wouldn’t look continuous to the observer.

Third, I don’t think the clock next to the safe would keep the safe safe (yeah I went there). The idea is that you couldn’t even ‘perceive’ a time difference. The light from the clock never looks slow to the observer, which is why it is a “space-time” cloak.

Now I would just like to go off on a semi-tangent of how I think the robbery would actually go down. It really involves just slow light. First you walk into the bank with a large thing of this material. People stare at you awkwardly, they see a ‘video’ of where the material was 10 minutes ago. You place this in front of the safe and walk away for 10 minutes. The material gets into the transient state. Then when nobody is looking, you jump over to the safe and do your thing. 9 minutes later you got the cash, you walk out nonchalant, because the safe still looks okay. At 9 minutes and 59 seconds, you active some device that cripples the meta material and the slow light scatters incoherently all over the room. Nobody saw you actually breaking into the safe, just being all awkward beforehand, and now people see the safe is empty.